1. Field of the Invention
This invention relates to an internal shoe-drum brake, primarily for a motor vehicle, of the general type including a pair of arcuate brake shoes which may be separated into braking engagement with a surrounding brake drum alternatively by a first actuator, normally hydraulically operated, for normal service braking, or by a second, normally mechanical, actuator for parking purposes.
2. Description of the Prior Art
One configuration of drum brake of the aforesaid general type is widely used for the rear brakes of passenger cars and incorporates a hydraulic service actuator and a mechanical parking brake actuator which are arranged to operate in the leading/trailing manner for both the hydraulic service and the mechanical parking actuation modes, being generally satisfactory for such vehicles in both modes of operation. However, for somewhat heavier duty applications, such as on light commercial vehicles for example, it is desirable to have a high factor parking brake, preferably of the duo-servo kind, in order to fulfil the heavier duty braking requirements for such vehicles.
One prior proposal of this latter kind is illustrated in U.S. Pat. No. 3,708,044, which illustrates a drum brake in which a hydraulic actuator and an automatic strut adjuster are both positioned in the aforesaid upper region and the mechanical actuator is positioned in the lower region of the brake. The mechanical actuating force is applied directly to one shoe and via a generally horizontal link to a first bell-crank lever pivoted to one end of the other shoe, the force being transferred by a link from the first lever to another bell crank lever pivoted at the other end of said other shoe so that the actuating force is applied more or less simultaneously to both ends of the shoe. Not only is this arrangement excessively complex, but the fixed points of force application to the shoes will not necessarily lead to an optimum duo-servo effect. Moreover, since the levers and links are mounted wholly at one side of the shoe, the input forces from the mechanical actuator are offset from the plane of the shoe web, leading to lack of balance and possible unsatisfactory operation in the duo-servo mode. Also, since the levers are pivoted on the shoes, these and the link will require at least some measure of dismantling during servicing of the brake, which is inconvenient and time consuming.